Hi Helge ,
Thank you for the kind words — and for the clear bug report!
You found a real issue: in the block editor the plugin called wp_doing_rest(), which is not a WordPress core function. That caused the fatal error when opening the editor. The frontend kept working because that code path was never reached there.
This is fixed in version 1.2.1. Please update the plugin and you should be able to edit pages normally again.
Sorry for the trouble, and thanks again for trying the plugin (and for considering PRO for your client project). If anything else comes up, just reply here.
Best regards,
CartEngine team
